Radio pulse-echo locator system to distinguish between moving and stationary objects



Dec. 13, 1949 R. S. HOLMES RADIO PULSE-ECHO LOCATOR SYSTEM TODISTINGUISH Filed Feb. 24, 1944 BETWEEN MOVING AND STATIONARY OBJECTS 2sheets-sheet 1 Nrolg.

nimm/5r Dec. 13, 1949 R. s. HOLMES 2,491,450

RADIO PULSE-ECHO LOCATOR SYSTEM TO DISTINGUISH BETWEEN MOVING AND`STATIONAI OBJECTS Filed Feb. 24, 1944 2 Sheets-Sheet 2 IN V EN TOR.

Paten ec. 13, 1949 UNITE STATES, P

RADIO PULSE-ECHO DISTINGUISH BE STATIONABY OBJECTS Loca'roa SYSTEM 'ro'rwEnN MOVING AND Ralph s. Homes, hmmm, N. s., a, Radio Corporation oi'America, a corporation of Delaware appresa@ February 24, 1944, serai No.52ans s claims. (ci. ass-s) My invention relates topulse-echo lcator anddistance determining systems and particularly to systems of this typedesigned for detecting and locating moving objects or targets such asapproaching aircraft.

One of the diiiiculties in locating an approach- A ing aircraft, forexample, is that Amany echo pulses are received as a result of reectionfrom stationary objects such as buildings, hills, etc. which tend tomask or obscure the pulses reilected from the aircraft. This isparticularly true ii' the aircraft is flying at a low altitude. YBecause oi this masking effect of undesired re-A A still further objectof the invention is` to f provide improved radio locator receiving meansfor locating moving targets.

In one preferred embodiment of the invention the system may include apulse transmitter and a pulse receiver of the type commonly employed inpulse-echo locator systems. In accordance with the present invention,the receivlng means also comprises a storage tube or the like by meansof which received pulses may be stored and then taken oi at a laterinterval. The storage tube includes a storage screen comprislng aplurality of capacity elements. Switching means are provided by means ofwhich re- :eived signals are first applied witha certain polarity to thestorage tube for an interval T1 i a few seconds, for example, and then,after i waiting interval during which no signals are lpplied, are againapplied vto thestorage tube mt with reversed polarity ior an intervalT2, prefrably of the same duration as the rst interval.

During the interval T1, charges will build up on pulse generator Il.

2 are those from moving mentioned "waiting period.

The invention will the following description taken in connection withthe accompanying drawing in which Figure 1 is a blockA and circuitdiagram of one embodiment ot the invention. Y

tollisfure 2 is a group of graphs that are referredIFigui-e3isaviewofastoragetube ofthetype that may be employed in thesystem of Fig. 1, and Figure 4 is a graph showing the switchingselruence for the storage tubein the systenrof pulse-echo systemcomprising a radio pulse transmitter i0 which is modulated by the pulsesfrom a pulse generator H whereby pulses of radio energy are radiatedtoward re#- Fig. 1 shows a fleeting objects or targets to be located.After reilection from a target, the pulses are received and demodulatedby a receiver I2 and supplied to a conventional cathode-ray tubeindicator represented by the block I8. Cathode ray de ection for theindicator I3 is provided by a sawtooth wave generator is which' issynchro nized by pulses supplied over a line i8 from the The system ofFig. 1 described thus far is conventional.

According to a preferred embodiment oi' the present invention the outputoi the receiver I2 is each storage screen element in proportion to thetrength oi' the echoes present at the range prresponding to thatelement. During the inter- 'al T2, the charge on eachelement will buildlp in inverse proportion to the signal present at hat range. Theresulting stored signals are' then aken oil' the storage tube foroperating an inicating device and the cycle is repeated. As will beexplained hereinafter, the signals that ave been reected from stationaryobjects are alanced out in the storage tube so that the only gnals thatappear in the storage tube output as the joint invention oi Harley ertalso supplied over a conductor Il to the "put-on beam control electrodeor grid I8 of a cathode' ray storage tube I9. By means of a switch 2iand a. polarity reverser tube 22, the received pulses are applied to thegrid IBArst with one polarity and then, after a waiting interval, withthe opposite polarity. Next, the closing oi a cam operated switch 23causes the stored signal to-be taken od and supplied through an outputresistor 24 to a second cathode-ray tube indicator 26.` This cycle ofput-on, "wait, reverse put-on, and ii is illustrated in Fig. 4.

Fig. 2 illustrates how this cycle of operation' to balance out except- Yfor the signal obtained from causes all the stored a' .f L;

a moving target. Before describing the circuit and circuit opera` tionmore in detail, reference will be made to flf.. 3 which illustrates onesuitable design for the storage tube I9, which is described and claimedin application Serial No. 492,658, iiled June 26, 1948,

Rose and f y L. Krieger. now 'Patent 2,454,652, issum No 23, 1948, andenti targets mueve changed their location substantially during theabovebe better understoodfromin1 explaining the operation of the systemof Cathode ray storage tube. The tube I9 comprises a highly evacuatedenvelope having a bulb portion 30, in which a storage screen 3| ismounted, and having two neck portions 32 and 33 in which are mounted ahigh velocity beam put-on" gun and a low velocity take-off" gun,

. respectively.

' The high velocity and low velocity electron guns may be of well knowntypes such as those employed in the high velocity beam and lowv velocitybeam'television pick-up tubes known as the Icon'- oscope `and vtheOrthicon, respectively. In the example illustrated, the high velocitygun comprises a cathode 36, the control grid I8, a-flrst anode 39 .and asecond anode 40. A collector trons from the storage screen 3|. Verticaldeiiection oi' the put-on beam may be produced by means of a pair ofdefiecting plates 42. Opersting voltages that may be applied to thevarious tube electrodes are indicated on the drawing merely by way ofexample.

lelectrode 4| is provided to collect secondary elec- Thestorage screen3|may be constructed' in various ways. One of the preferred constructionscomprises a supporting sheet of mica. 43 about 0.001 inch thick whichhas conducting strips or lines 44 extending horizontally across itsfront surface. The strips 44 may be formed, for example, by `sputteringa conducting coating of platinum upon each side of 'the mica sheet andthe front or scanned'side with a ruling machine to separate the metalinto conducting lines. The metal coating` 46 on the baci; side of themica sheet is used, in the example illustrated, as the signal plate fortaking ci! the signal through the output resistor 24.

During the processing of the tube, the screen 3| preferably hassilverevaporated thereon to make the secondary emission uniform over thewhole line surface. The amount of silver introduced is too small to makethe space between the then scratching about 100. lines to the inch online 44 conducting. A small amount of caesium 1 may also be introducedinto the tube during the processing to make sure that the ratio ofsecondary electrons to primary electrons is greater than unity. Since itis preferred that the screen 3| shall not be photoelectric, no oxidizingstep preceding the introduction of caesium is required,

as would be the case in processing a mosaic screen for an Iconoscope.

The screen 3 I, when mounted in the tube, preferably is curved toconform to the surface oi' a cylinder having an axis that passesthrough'or near the centers of deection of the two 'electron beamswhereby the point of smallest beam diameter is always at the screensurface as the beam is deilected and vwhereby the beams during bombardedstrips 44 are given a more positive potential.

At the other end of the screen .3| the conduct; ins strips 44 arescanned during the take-oit" .period (F18. 4) by deilecting the lowvelocity beam vertically and across the strips 44 by means of deflectingcoils 50, for example. Except during the take-olf period, the lowvelocity beam is blocked by a high negative bias on the control grid 51.When the strips 44 are scanned by the low velocity beam, the electronsof the beam strike the conducting strips 44 with a velocity so that theratio of secondary electrons to primary or beam electrons is less thanunity. According to one method of operation, sufflcient beam current isprovided to return the conducting lines 44 to the potential of thecathode 56 o! the low velocity beam gun. Thus the signal put on by theirst beam is substantially completely wiped off" by a single scanning ofthe second beam. It may be preferred to operate the tube with a lowvelocity beam4 of less current density so that the beam removes onlypart of the stored signal as it scans once across the screen, butcompletely wipes oit the stored Signal during the several scannings thatoccur during the take-oil period. A

In the example illustrated, the low velocity beam electron gun comprisesthe cathode 5G, the control electrode 51, a first anode 53 and a secondanode 8|. Two frame-like or ring electrodes 62 and 63 are positionedbetween the second anode Il and the screen 3| for slowing down theelectrons after they leave the region oi the second anode 6|.

Referring again to the circuit of Fig. 1, the

'switch arm 2|a and th'e switch arm 23 (which is actuated by a cam 23')are driven in a fixed time relationby a motor 1|. The time relation ofthe several switch positiom will be apparent from Fig. 4, the switch arm2|a being in contact with a commutator segment a during the rst put-oninterval Ti, in contact with a oommutator segment b during the "waitingperiod, in contact with a commutator segment c during the second put-oninterval Ts, and in contact with a commutator segment d dln-ing thetakeoif period.

During the put-on'f interval T1 the "put-on beam is biased substantiallyto cut-orf or minimum current value by means of a biasing source such asa battery Il.'y Thus, the charge on the capacity armonie-elements of thescreen 3| at a particular point on the screen is increased from zero or,nearly sercyslueby any renected signal received during the period T1 ssshown by the graph 13. During the waiting interval, also, the put-onbeam is biased to cut-off by means of a battery 2l whereby there is nochange in the charge on the elements of screen 3| during the waitingperiod.

It may be noted that the left-hand portions of the graphs I8 and 13corresponding to distances near the transmitter and receiver represent acondition where the receiver ampliner is overloaded by the strong echoesfrom the ground and nearby objects.

During the next put-on" period T2, a bias is applied from a battery 25to make the put-on beam current of such value 'as to charge the capacityelements of the screen 3| to about onehali' their maximum charge in theabsence of a receiving signal.V The maximum charge referred to is themaximum assuming linear operation of the storage tube, that is,operation where control grid voltage vs. charge on the mosaic orcapacity elements of the screen 3| is linear. Y During the period Ta, areceived' signal cames less charge and upon various other factors.

atei,

tobe put on the mosaic elements at a particular point on the screen asshown by graph 18.

During the "take-oil" period, the "put-on beam-is again biased tocut-oir by the battery 20 whereby the charges on the elements of thescreen 3l are affected only during the two puton intervals T1 and T2.

At the start of the take-oil interval, the switch arm 23 is moved fromits left-hand position of high negative bias to its right-hand positionoi?4 low negative bias whereby the "takeoil beam is unblocked andremains unblocked during the "take-oil interval. As shown in Fig. 4, the"take-oir interval may be of comparatively short duration. The highnegative bias for blocking the take-off beam at the end of the"takeoiinterval may be provided by a battery 12 and the bias for take-oil beamopera tion may be provided by a battery 13.

Sawtooth generators 14 and 16 are provided for deflecting the take-offbeam of the storage tube and the cathode ray or beam of the indicatortube 26 in synchronism. The deiiection rate for these beams may be thesame as the pulse rate of the transmitted pulses and the generators 14and 16 may be synchronized by a lead 11 from the pulse generator Il asshown, or if preferred, a lowex deection rate may be employed.

The operation of the invention will be apparentirom the foregoingdescription and from an inspection of the graphs in Fig. 2. The inputsignals applied during the irst and second puton" intervals Ti and T2,respectively, are stored on the capacity elements of the storage screen3| with complementary values, as shown by the graphs 18 and 19. Duringthe second put-on interval, the pulses reected from xed targets arriveat the receiver, which is assumed to be stationary or substantially so,at a time following the instant of pulse transmission that is the sameas their time of arrival during the rst put-on period. Therefore, thesepulses from stationary objects are stored on the same screen storage.strips or elements during both put-on periods. The effect is that, as tostationary objects, the signals received during the two periods add onthe storage elements so that they are brought to a common voltage levelas shown by the graph 8| of Fig. 2.

As to a moving target, however, the waiting period is long enough sothat the signals reilected from it are stored at dierent points on thescreen 3l so that they do not cancel each other. Therefore, the movingtarget signal, and this signal only, appears in the storage` tube outputas illustrated by graph 8l of Fig. 2. 'Ihe resuit is that the movingtarget signal is visible on the screen of the cathode ray tube 26 eventhough there may be enough reiiected signals from iixed Vobjects to maskit in the absence of cancellation or xed target signals.

It will be understood that the actual duration of the several intervalsin the cycle of operation, as well as their relative duration; willdepend upon the use for which the apparatus is designed The speciictiming indicated in Fig. 4 is given merely as one example of a suitabledesign for a pulse-echo system having about a 100 mile range andtransmitting comparatively wide pulses such as pulses of l0 microsecondsduration. In this example, the resolution of the system is about onemile and the waiting period has been made long lenough, 36 seconds, topermit an. airplane to travel about two miles, lassuming its speed isabout 200 miles per hour. During each "put-on period o f 9 seconds, theairplane will have time to travel only one-half mile. Since this is lessthan the one mile resolution of the system, the movement of the airplanewill not appreciably aiect the sharpness of the moving targetindication.

I claim as my invention:

l. i radio system comprising a pulse transmitter for radiating radiopulses. a pulse-echo receiver, said receiver including means i'orreceiving said pulses after they have been reected from reiiectingobjects, and including a storage tube having a storage screen comprisingstorage elements. means for storing said reiiected pulses on saidstorage screen on certain of said storage elements at points along atime axis thereon f which correspond to the distances of the reiiectingobjects, means for preventing received reiiected pulses from -beingstored during a waiting period that is substantially longer than theduration of each of said pulses, means for receiving pulses reilectedfrom said objects after said waiting period and for storing them on saidscreen at points along a time axis thereon which correspond to thedistances of the reiiecting objects and with the storage being on saidcertain storage elements when the reflected pulses are from stationaryobjects, and means for -balancing the later received pulses against saidstored pulses whereby the pulses reected from moving objects aresubstantially the only pulses thatare not balanced out.

2. In a radio locator receiver for a system of the type having a radiotransmitter that transmits radio signals toward reiiecting objects, astorage device for storing electrical signals, said device including astorage screen comprising storage elements, means for receiving saidradio signals after they have been reilected from said reiiectingobjects, means ior storing received signals on said storage screen oncertain of said storage elements during a certain time interval and atpoints along a time axis thereon which correspond to the distances ofthe reiecting objects. means for again storing received signals on saidstorage screen but .with reversed polarity after a waiting interval andat points along a time axis thereon which correspond to the distances ofthe reflecting objects and with the storage being on said certainstorage elements when the reiiected pulses are from stationary objectswhereby the signals reflected from stationary objects are substantiallybalanced out in said storage device, and means for taking signal oilsaid storage device at the end of the storage of the reversed polaritypulses to obtain an indication of moving objects only.v

3. In apulse-echo receiver for a system of the type having a radio pulsetransmitter that transmits radio pulses toward reilecting objects, astorage device for storing received pulses that have been reflected fromsaid reiiecting objects, said device including a storage screencomprising storage elements, means for storing said pulses on saidstorage screen during successive spaced time intervals that are longcompared with the pulse period with the polarity of the pulses appliedto said device during one interval opposite that of the pulses appliedto said device during the next successive interval and at points along atime axis on said storage screen which correspond to the distances ofthe reflecting objects, said la'st means including .means for storingsaid pulses on the same storage elements during two successive intervalswhen the pulses are being reilected from stationary objects,` and meansfor taking signal oi said storage device at the end of two succesannuocathode-ray storage tube of thetype comprisingv V a storage screenhaving capacitive storage elements, means for receiving pulses afterthey have been reilected from said reflecting objects, means for storingreceived pulses on Asaid storage screen on certain of said storageelements during a certain time interval and Aat points along a time axisthereon which correspond to the distances o! :the reilecting objects,means A for again storing recelved pulses en said screen but withreversed po- I larity after a waiting interval and at points along atime axis thereon which. correspond to the distances of the reflectingobjects and with the storage being on said certain storage elements whenthe reilected pulses are from stationary objects whereby the pulsesreilected from stationary objects are substantially balanced out on saidstorage screen, and means for taking signal off said screen at the endofthe storage of the reversed polarity pulses to obtain an indication ofmoving objects only.

5. In a receiver for a pulse-echo radio locator system of the typehaving a radio pulse transmitter for transmitting electrical pulsestoward` reflecting objects, a cathode-ray storage tube of the typecomprising a storage screen having capacitive .storage-elements, meansfor receiving said electrical pulses after they have .been reilectedfrom said reflecting objects, means for storing a plurality of receivedpulses on said storage screen on certain of said storage elements duringa storage time interval and at points along a time axis thereon whichcorrespond to the distances of the reflecting objects, means for againstoring a plurality of receivedpulses on said screen during a secondstorage. time interval but with reversed polarity after a waitinginterval which islonger than either of said storage time intervals andat points alongv a timeaxis thereon which correspond to the distances ofthe reilecting objects and with the storage being on said certainstorage elements when the reflected pulses are' from stationary objectswhereby the signals, reilected from stationary objects are substantiallybalanced out on said storage screen, and means for taking signal oilsaid screen at the end of the storage o! the reversed polarity pulses toobtain an indication of moving objects only.

6. -In a receiver for a pulse-echo radio locator system of the typehaving a radio pulse transmitter for transmitting electricalpulsestoward reilecting objects, a cathode-ray storage tube o!v the reiiectingobjects but which are reversed in y polarity and with the storage beingonsaidv certain storage elements when the reilected pulses are fromstationary objects whereby the .signals reilected from stationaryobjects are substantially balanced out on said storage screen, and

means for taking signal oi! said screen at the end of the storage of thereversed polarity pulses to obtain an indication of moving objects only.

'7."In a pulse-echo system that includes areceiver having a storage tubecomprising va storage screen having storage elements,A the method oi vreception which comprises receiving pulses that have been reilected fromreflecting objects, storing the pulses that are received during acertain time interval on certain of said storage elementsat points alonga time axis on said screen which correspond to the distances of thereilecting objects, waiting for a time longer than said certain timeinterval without storing pulses, and again storing onsaid screen pulsesthat are received during a certain timeinterval and at' points along atime axis thereon which correspond to the distances of the reilectingobjects but with the polarity of the pulses reversed for balancing themagainst the first storedpulses and wth the storage being on said certainstorage elements when the reflected pulses are from stationary objectswhereby pulses reflected from moving objects are substantially the onlypulses that are not balanced ou 8,. In a receiver for a pulse-echo radiolocator system of the type having a radio pulse transmitter fortransmitting electrical pulses toward reilecting objects, a cathode-raystorage tube of the type comprising a storage screen having capacitivestorage elements, said tube having an electron gun for producing aput-on beam and an electron gun for producing a take-off beam, means fordeilecting said beams. means for receiving said electrical pulses afterthey have been reflected from said reflecting objects, means forintensity modulating the put-on beam in a direction to increase the beamcurrent to store a plurality of received pulses on certain of saidstorage elements of said storage screen during a storage time intervalat points along a time axis thereon which correspond to the distances ofthe reflecting objects, means for intensity modulating the put-on beamin a direction to decrease the beam current to store a plurality ofreceived pulses on said screen during another storage time interval atpoints along a time axis thereon which correspond to the distances ofthe reflecting objects the type comprising-a storage screenhavingcapacitive storage elements, means -for receiving saidelectrical'A pulses after theyhave been reilected from said reflectingobjects, means for and with thestorage being on said certain storageelements when the received pulses are from stationary objects, said twostorage time intervals being separated by a waiting interval, means forbiasing the put-on beam gun substantially to beam current cut-oi! duringthe storage interval during which the modulation increases the beamcurrent, means for biasing the put-on beam gun to make the put-on beamcurrent approximately one-half maximumY beam current during the otherstorage time interval, means for biasing the put-on beam gunsubstantially to beam current cut-off during said waiting intervalmeansfor blocking said take-oi! beam during said put-on and waitingintervals, means for blocking said pnt-on beam at the end of the secondput-on period, and means for simultaneously unblocking said take-oi!beam and taking signal off said slrleen to obtain an indication ofmoving objects 0 y- 9. In a receiver tor a-pulse-echo radio locatorsystem of the type having a radio pulse transmitter for transmittingelectrical pulses toward refleeting objects, a cathode-ray storage tubeof the type comprising a storage screen having capacitive storageelements, said tube having an electron gun for producing a put-on beamand an electron gun for producing a take-off beam, means for deflectingsaid beams, means for receiving said electrical pulses after they havebeen reflected from said reflecting objects, means `for intensitymodulating the put-on beam in a direction to increase the beam currentto store a plurality of received pulses on certain of said storageelements of said storage screen during a storage time interval at pointsalong a time axis thereon which correspond to thedistances of thereflecting objects, means for intensity modulating the put-on beam in adirection to decrease the beam current after a Waiting interval to storea plurality of received pulses on said screen during a second storagetime interval at points along a time axis thereon which correspond tothedistances of the reflecting objects and with the storage being on saidcertain storage elements when the received pulses are from stationaryobjects, means l for biasing the put-on beam gun substantially to beamcurrent cut-oil during said first storage interval, means for biasingthe put-on beam gun to make the put-on beam current approximatelyone-half maximum beam current during said second storage time interval,means for biasing the put-on beam gun substantially to beam currentcut-01T during said Waiting interval, means for blocking said take-offbeam during said put-on and waiting intervals, means for blocking saidput-on beam at the end of the second put-on period, and means forsimultaneously unblocking said take-off beam and taking signal off saidscreen to obtain an indication of moving objects only.

RALPH s. HOLMES.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,208,349 Ulbricht July 16, 19402,430,038 Wertz Nov. 4, 1947

